Effect of Environmental Conditions on Copepod Size Structure and Energy Flux in the Pelagic Ecosystem of the Kuril Islands Region

Spatial and vertical variations in copepod abundance, carbon biomass, and size spectra are analyzed to elucidate the effects that the highly variable oceanographic conditions of the environment around the Kuril Islands exerted on the copepod size structure and energy flux in the pelagic ecosystem during the summer of 2024. Three water masses have been delineated based on hydrological characteristics: the subarctic Oyashio Current Water (OCW), the subtropical Soya Warm Current Water (SWCW), and the Kuroshio Extension Water (KEW). The total abundance of copepods decreased from OCW to KEW (OCW > SWCW > KEW), while the total carbon biomass did not differ between the OCW and SWCW but was reduced in the KEW. Clear vertical patterns of variations in average carbon biomass have been recorded: the highest peak in the SWCW, driven by medium-sized copepods above the thermocline and by large-sized copepods below it; an intermediate peak in the OCW, characterized by the dominance of large-sized copepods throughout all layers; and the lowest peak in the KEW, with small-sized copepods dominating above the thermocline and medium-sized copepods below it. Normalized abundance size spectra (NASS) and normalized carbon biomass size spectra (NBSS) were constructed for each water mass and sampling layer. The copepod productivity declined towards higher latitudes, while energy transfer efficiency and size structure stability showed a tendency to increase with lower water temperatures and with depth. The combination of chlorophyll a and temperature was the primary factor controlling the copepod accumulation above the thermocline, whereas the combination of dissolved oxygen and chlorophyll a was the primary factor below the thermocline. This study highlights the significance of each water mass to the changing pelagic ecosystem in the northwestern Pacific Ocean.